Polyurethane solutions or dispersions in the form of a water-dilutable aqueous or predominantly aqueous phase are known by the expert as HEUR thickeners (the acronym HEUR derives from “nonionic hydrophobically modified ethylene oxide urethane block copolymer”) and have been used for some time now for thickening water-based emulsion paints in various fields of application.
The HEUR thickeners described at the end of the 70s in U.S. Pat. No. 4,079,028 are made up of linear and/or branched polyethylene glycol blocks and hydrophobic segments which are generally linked together by urethane groups (or urea groups where amines are used instead of alcohols).
The thickening effect of HEUR thickeners is assumed to be attributable to the fact that the polyethylene glycol segments ensure compatibility with water while the hydrophobic segments build up a viscosity-generating three-dimensional molecular structure in the emulsion paint to be thickened through an association with one another and with dispersed binder particles of the emulsion paint.
Preferred hydrophobic segments in commercially available HEUR thickeners are relatively long-chain, generally monohydric alcohols, such as for example n-octanol, n-dodecanol, isotridecyl alcohol, isononylphenol or ricinoleic acid methyl ester. These alcohols are predominantly used as such, but also in the form of their addition products with a few equivalents of ethylene oxide.
The polyfunctional isocyanate units predominantly used in commercially available HEUR thickeners are generally difunctional and include, for example, methylene-bis-(4-cyclohexyl)-diisocyanate, m/p-tetramethylene xylylene diisocyanate, hexamethylene diisocyanate, 2,4-toluene diisocyanate, trimethyl hexamethylene diisocyanate or 4/2,4′-diphenylmethane diisocyanate. The polyethylene glycol segments used in commercially available HEUR thickeners are generally also difunctional and have molecular weights of a few thousand dalton, for example 4,500 or 10,000 dalton.
The ratios between the individual constituents of HEUR thickeners, i.e. branched or unbranched polyethylene glycols, mono- or polyfunctional hydrophobic alcohols, pre-ethoxylated mono- or polyfunctional hydrophobic alcohols, chain-extending di- or polyfunctional short-chain alcohols, are generally selected so that one hydrophobic alcohol is available to each ethylene glycol segment end still reactive through a hydroxyl group.
The hydroxyl-terminated synthesis units of HEUR thickeners are joined together by reaction with di- or polyfunctional isocyanates, the equivalent ratios of the isocyanate groups to be reacted with one another in an addition reaction and the “H-acidic” groups (generally OH groups, although they may also be NH2 groups) being selected so that, for every “H-acidic” group equivalent, i.e. generally every OH group, there is at least slightly less than one isocyanate group equivalent. In other words, the equivalent OH:NCO ratio is generally adjusted to a value of at least 1:1, a ratio of 1:1 being ideal or the OH groups outweighing the NCO groups by 5-10% (which corresponds to an equivalent OH:NCO ratio of 1.05:1 to 1.1:1) in order to ensure that the end product (the HEUR thickener) does not contain any free NCO groups which are undesirable on the one hand for toxicological reasons and, on the other hand, because they are capable of entering into unwanted secondary reactions with formulation ingredients when the thickeners are subsequently used in formulations to be thickened. This basic principle, i.e. that, in the production of HEUR thickeners, the OH groups of the polyethylene glycol and hydrophobic alcohol segments slightly outweigh the NCO groups of the isocyanate segments by ca. 5-10%, is also part of the teaching of the above-cited U.S. Pat. No. 4,079,028 (cf. column 3, lines 17 et seq).
HEUR thickeners have recently been acquiring increasing interest as thickeners for disperse cosmetic preparations (cf. for example EP-A 787,486).